This invention relates to a prosthetic hand and more specifically to an anthropomorphic hand providing for a conformal, compliant grip and having an opposable functional thumb. (This is the patent offices preferred method of description.)
From the time the first human survived an amputation, man has tried to make replacement body parts. The first replacement for an upper extremity amputation that seemed to be useful was the famous hook. It allowed the amputee to hold an object down, and pull it to them. If the object could be stabbed without ruining it, the amputee could pick up the object. The hook was slightly more useful than just using the stump.
The next successful prosthesis is the split hook. There have been many specific designs for the split hook, but they are functionally the same. Most are cable operated, and can be either voluntary opening, or voluntary closing. There have also been many methods of operation. U.S. Pat. No. 5,219,366 is an example of the split hook, with a novel operating system. U.S. Pat. No. 5,219,366 may look much different, but in basic terms is still a split hook. The split hook has an advantage over the hook in that the two halves of the hook can grasp an object and pick it up without having to resort to stabbing the object. Split hooks do allow a good view of the object being picked up. U.S. Pat. No. 5,219,366, while having several grasping surfaces, lost the ‘good view’ quality of the split hook. The biggest drawback to the split hook is the limited range that the hook can be opened in order to grasp an object. The split hook is not able to pick up a glass unless part of the hook is inserted into the glass, and whatever the glass contains. U.S. Pat. No. 4,149,278 is an example of the split hook that is operated by an electric motor. This patent also includes a ‘wrist rotation’ unit allowing the hook to be more easily aligned to the object being picked up. However it is still just a split hook. There have been the odd designs that could be termed the ‘Swiss Army Knife’ of the split hooks. These are still basic split hooks, though sometimes several split hooks in one unit. These numerous units would supposedly allow more usability. U.S. Pat. No. 4,332,038 is an example of such a ‘multi-tool’ design. These may gain a slight improvement in usefulness. However, they loose the advantage of the ‘clear view of the object’ allowed by the split hook. They also are so bazaar in appearance that very few amputees would ever consider using them in public.
Another variant that came from the split hook is the claw. U.S. Pat. No. 4,225,983 is an example of the claw design. The claw design usually has a wider opening range, allowing larger objects to be picked up. The shape also reaches around round objects to hold them. This prevents the object from slipping out of the wedge force exerted by a split hook. These also allow a good view of the object being picked up. They still have the same usability problems that the split hook has. Objects with a complex, or a tapered shape can not be held securely, and sometimes not at all. There have also been variants of the claw for specific purposes. One such is U.S. Pat. No. 5,163,966 for holding round bar materials. U.S. Pat. No. 4,377,305 is an example of the claw that is common for robotic use. This is virtually useless for prosthetics. The overall bulk of the claw would prevent most amputees from using it, and again, tapered objects can not be held securely. U.S. Pat. No. 4,990,162 is another example of the variations on the claw. This one in particular would probably damage most objects it attempts to pick up. The overall appearance would also keep most amputees from even considering it for use. U.S. Pat. No. 5,800,572 is a variation that has bigger drawbacks than most claws. There is no opposition contact possible with the two halves of the claw. The objects being picked up would have to be long enough to span all three ‘fingers’ of the claw to be picked up. U.S. Pat. No. 5,013,326 has the three ‘fingers’ spaced so that the single ‘thumb’ digit makes contact with both of the ‘finger’ digits. This design is in use in prosthetics for children. Like the others, it is very limited in what it is able to grasp.
The next progression in the anthropomorphic hand is fingers that flex at all the joints. This allows greater surface contact with an object being picked up. The force required to hold the object is reduced, reducing the battery power needed for electric hands, and less cable tension required by conventional (body powered) hands. U.S. Pat. No. 5,326,369 shows one method of bending jointed fingers. Driving a rotating force with a cable is a poor choice when the cable is bent while driving the force. The cables tend to develop torque induced twist that will cause the ‘externally threaded cinctures’ to bind in the bushings as well as the destruction of the cable. The sections of the fingers will also be moved in a given ratio to the others. The fingers will not be able to conform to the object being grasped. This will result in a small surface contact area being used for any object that does not perfectly fit into the designed curve of the fingers. This requires the force needed to grasp the object to be much higher, like that of a claw. U.S. Pat. No. 5,941,914 does not have the problem of the driving force causing binding, but the fingers are still locked into a designed curve of movement, with each section moving proportionally to the other sections. Again, unless the object just happens to fit that designed curve, more force is needed.
U.S. Pat. No. 4,094,016 shows a full hand, with flexing fingers. The thumb is built to also flex, but is locked into opposition of the fingers. Both the fingers and the thumb are positioned by the rotation of a single cam. The digits are also moved in a ‘ratio’ that gives a predetermined curve to the fingers. This again provides no conformability to objects. U.S. Pat. No. 4,364,593 shows a similar hand. This hand uses a complex linkage to operate the fingers. If the fingers do not make contact with the object at the same time, this linkage design will not allow for the torque loads that would be placed on parts 65 and 66. When the first finger makes contact, the forces can bind these two parts. There would only be a very small surface contact area being used to hold the object. This could easily damage those parts, and parts 17, 27, 37, 47, and 57. This would render the hand nonfunctional. This hand still functions as a claw.
U.S. Pat. No. 5,080,681 has some improvement in its ability to function. By using springs to extend the fingers, and a flexible substance to flex the fingers, the fingers are compliant to external forces that would tend to flex the fingers. The use of a flexible material for the ‘tendons’, will allow the fingers to flex at differing joints to conform to an object, but they will both have the same amount of total flex, which can prevent full contact with the object being grasped. The use of two tendons is actually useless, as the cable that operates one, operates the other in the exact amount. The ‘first sliding actuating member’ and the ‘tendons’ attached to it could be removed completely and the function of the hand would not be affected at all. No where in the description or claims, is there ever an explanation of how the sections work together to increase the functionality of the device. If there is a purpose for the second sliding member, and related tendons, the inventor forgot to include them, and an explanation of how it should work. This is a claw that has a slightly conformable grip.
U.S. Pat. No. 5,200,679 shows a hand that has conformability of the fingers. Unfortunately the fingers must either be operated by one motor, or by a motor for each finger. Using one motor, the hand would loose most of the conformability. This would occur when the first finger makes complete contact with the object being grasped. If separate motors are used for each finger, either complex control circuits must be built to allow each finger to continue to move until making contact, or several control channels would need to be used to control each finger separately. The other channels would have to be driven by separate myoelectric sensors, which is not plausible due to the limits of how many suitable sites can be found on the body that would not cause excessive cross talk interference between each channel. The cable system of this hand also has inherent problems. The fingers have no compliance to external forces that would cause the fingers to flex. The larger problem with the dual cable system has to do with excessive slack, or tension in the cables. Since one reel would be full and the other empty when the finger is fully flexed or extended, there is a difference in the movement of each cable, or each end of a single cable, for a given rotation of the shaft. This would either cause an excessive amount of slack, or tension in the cables. Excessive slack would allow the cables to slip off the reels, requiring the hand to need repair work. Excessive tension could cause damage to the cables, or the fingers, or the shaft and reels. This hand may well be usable for robotics, but is not satisfactory for prosthetics.
U.S. Pat. No. 4,986,723 also has the problem of too many control channels. It also has a very large number of parts that can fail, resulting in excessive maintenance. The cables are all kept under tension by springs. So they are less likely to come off the pulleys, but may slip on the pulleys, causing the sections of the fingers to get ‘out of sync’ with the other parts. This design also requires a large number of motors to operate it. Even if enough control channels were available for such prosthesis, the weight of the motors, and required power supply would prevent its use. On average, amputees will only tolerate prosthesis weighing less than 3.5 pounds. The motors alone would exceed that weight.
U.S. Pat. No. 5,080,682 has the problem also of too many motors needed to operate it, and the control channels needed to drive them. It also uses a flexible device (item 30) to push the digits into flexion. As it is easier to pull a chain than push it, this method of moving the fingers would fail, and be damaged at the slightest amount of pressure on the fingers. The ‘push rod’ (item 30) if flexible enough to bend around the fingers, would also be flexible enough to bend off line of its intended path, resulting in no movement of the finger, and possibly damage to the push rod. A flexible means of pushing a digit into flexion will structurally fail, but is completely acceptable to pull the digit into flexion.
U.S. Pat. No. 5,447,403 must be impressive in the lab, but will never exist as a functional prosthesis. It requires 16 servos to operate the hand, and 2 motors to operate the wrist. Again, there are too many control channels, and too much weight in the drive system. The hand would work well in a lab, or as part of a big, heavy robotic system.
U.S. Pat. No. 4,685,929 shows a voluntary closing hand that will function well. The thumb is adjustable to several positions, but is static while in use. The cable system is a good design, and the return spring system used to extend the fingers will work well. The fingers have compliance to external flexing forces, with out apparent damage happening. The fingers would each demonstrate a very slight degree of conformability. The design allows all fingers to be operated by one cable. The linkage has an offset to the lever moments to bias the strength to the first two fingers. A side effect of this will also allow a very slight amount of compliance in movement between the fingers, but very little before the linkage binds on itself. The lack of a functional thumb is a draw back. This hand, like many of the others, is still a nice looking claw.